This invention relates to plastic compositions containing a photosensitizer which renders the composition degradable by ultraviolet radiation.
The advent of plastics has given rise to improved methods of packaging goods. For example, polyethylene and polypropylene plastic films, bags, bottles, styrofoam cups and blister packages have the advantages of being chemically resistant, relatively unbreakable, light in weight and translucent or transparent. The increasing use of plastics in packaging applications has created a serious waste disposal problem. Burning of these plastic materials is expensive and may add to air pollution problems.
Unlike some other packaging materials, such as paper and cardboard, plastics are not readily destroyed by the elements of nature. Thus, burying them is not an effective means of disposal and can be expensive.
Plastics are biologically recent developments, and hence are not easily degradable by micro-organisms which attack most other forms of organic matter and return them to the biological life cycle. It has been estimated that it may take millions of years for organisms to evolve which are capable of performing this function. In the meantime, plastic containers and packaging films are beginning to litter the countryside after being discarded by careless individuals.
One means suggested for combating the plastic waste disposal problem has been the use of plastic compositions which degrade upon exposure to ultraviolet light. This approach would take advantage of the natural tendency of most organic polymers to undergo gradual reaction with atmospheric oxygen, particularly in the presence of light. Typically, a photosensitizing additive is employed in order to accentuate this natural tendency. The photosensitizing additive absorbs ultraviolet light (e.g., from sunlight); the additive, in the resulting photo-excited state, undergoes a chemical reaction which leads to the generation of free radicals which initiate an autoxidation process thereby leading to the eventual disintegration of the plastic material.
According to the understanding of the state of the art, sensitized photo-oxidative degradation processes involve the photochemical generation of free-radical intermediates, followed by subsequent reactions which are essentially thermal (i.e., nonphotochemical). The photosensitizing additive increases the rate of formation of free radicals, but does not appreciably alter the rates of subsequent reactions of those radicals. Consequently, the rate of the overall process, which may be quite rapid initially, can decrease drastically if the sensitizer is consumed during the reaction, or if the source of light is removed.
The present invention provides further control of the rate of sensitized photodegradation through the use of a second type of additive, referred to hereinafter as a "readily autoxidizable organic substance", in addition to the photosensitizing additive.
The primary function of this "readily autoxidizable" substance is to accelerate those "thermal" autoxidation steps which follow the initial photochemical initiation step; the presence of the readily autoxidizable substance may also increase the rate or efficiency of photochemical initiation.